In a way, there are two parts to this Idea. The first part is about the coin.
Imagine a pure-gold coin that weighs 10 grams (about 1/3 of a troy ounce).
Embossed on the coin would be a description of that: "24K gold, 10 grams".
There might be some design or other on the coin, but they are not
as
important
as the description. If you took the coin to a store, modern computer tech and
Internet connectivity could easily let both you and the store-owner know the
current commercial valuation of the coin. If it happened to be worth $300,
then
you could buy something that cost $300 with it. If it happened to be worth
$500,
then you could buy a $500 item instead.

Now imagine other coins made of other substances, such as silver,
palladium,
iridium, and platinum. None of them would be given a value-denomination; all
would simply be marked with descriptions of the substance, and the
magnitude
(like "100 grams").

But there is no reason to only have high-value-metal coins. Copper, of
course,
has long been used for low-value coinage. So have nickel and tin and
aluminum.
But we could widen the list considerably, by including other metals that are
neither especially common nor especially rare: chromium, molybdenum,
zirconium, titanium, vanadium, niobium, gadolinium, tantalum, tungsten, and
so
on, even including lead. Collect the whole set! We would NOT want to use
metals like uranium, but not because they are radioactive (depleted uranium
is
low-level enough to be reasonably safe, while others are certainly deadly
dangerous), but because they simply don't stay PURE, a side-effect of their
radioactivity.

Before we get to the second part of this Idea, we need to cover an
"intermediate"
sort of thing. Historically, a problem with high-value coinage was "clipping"
(or
something equivalent). Take a bunch of gold coins and put them together in a
sack. Then put the sack onto a shaking device, and let it shake for a month
or
two. Open the sack, remove the coins, and note a fair amount of gold dust is
now in the sack. Keep the dust and pretend the coins have the same value
as
when you obtained them....

NOT FAIR! We want these coins to be trusted to actually be what they say
they are. Obviously any place that is willing to accept these coins is going to
need a fairly accurate scale, to be sure the coin actually weighs what it says
it
weighs. And an "X-ray spectrometer" would be a nice thing for the merchant
to
have, too (see link). It will verify that the substance of the coin is what it says
it
it is, and do so non-destructively.

The second part of this Idea has a two-fold purpose, one of which can partly
assist the intermediate thing just described. Seal each coin in a thin clear
plastic case. If the case is damaged, that could be a sign that someone
tampered with the coin. It would certainly be a reason to take the coin to a
minter, where it could be put into a new case after thorough analysis (or
physically ripped into atoms, for separation into whatever elements it actually
happened to be made of).

The second purpose of the plastic case relates to the reason why coins made
of
iron generally were not done. A number of metals from which coins might be
made will seriously corrode when exposed to the atmosphere. The plastic
case
seals them away from the air, and thus the coin remains pure. As a result,
we
could deliberately make coins from some of the more reactive metals, like
rubidium. Anyone who opens THAT case, trying to devalue the coin, will be
in
for a surprise!

I suspect the original coins were like that. Little ingots, standardized by the issuing authority. I suspect you could use a gold bullion coin exactly as you describe. Bring it into a pawn shop and say you want to trade it for something of value. The shop will assess how much the coin is worth, test it to see if it is real and then make you an offer.

Well, in Archaic Greece, obols were actually metal spikes, and a "drachma" meant a "handful" (of metal spikes) - specifically, six of them - which gives you a clue to either (a) how big the spikes were or (b) how big an Archaic Greek fist was, but not both.